C Program to Find Factorial using Recursion

This program calculates the factorial of a number using recursion in C. It demonstrates how a recursive function can call itself until a base condition is reached.

Concept Overview

A factorial of a number n (written as n!) is the product of all positive integers less than or equal to n. Using recursion, the factorial of n is defined as: n * factorial(n - 1), with factorial(0) = 1.

Program

#include <stdio.h>

// Recursive function to calculate factorial
unsigned long long factorial(int n) {
    if (n == 0 || n == 1)
        return 1;
    else
        return n * factorial(n - 1);
}

int main() {
    int num;
    printf("Enter a number: ");
    scanf("%d", &num);

    if (num < 0)
        printf("Factorial is not defined for negative numbers.\n");
    else
        printf("Factorial of %d is %llu\n", num, factorial(num));

    return 0;
}

Output

Factorial of 12 is 479001600

Explanation

• The function `factorial()` takes an unsigned integer as input.
• If the input `i` is less than or equal to 1, the function returns 1 — this is the base case.
• Otherwise, it returns `i * factorial(i - 1)`, calling itself recursively.
• In `main()`, the factorial of 12 is calculated and printed.